Protein kinases represent a large family of proteins that play a central role in the regulation of a wide variety of cellular processes. Through regulating an array of signaling pathways, protein kinases control cell metabolism, cell cycle progression, cell proliferation and cell death, differentiation and survival. There are over 500 kinases in the human kinome, and over 150 of these have been shown or are proposed to be involved in the onset and/or progression of various human diseases including inflammatory diseases, cardiovascular diseases, metabolic diseases, neurodegenerative diseases and cancer.
A partial list of such kinases include abl, AATK, ALK, Akt, Axl, bmx, bcr-abl, Blk, Brk, Btk, csk, c-kit, c-Met, c-src, c-fins, CDK1, CDK2, CDK3, CDK4, CDK5, CDK6, CDK7, CDK8, CDK9, CDK10, cRaf1, CSF1R, CSK, DDR1, DDR2, EPHA, EPHB, EGFR, ErbB2, ErbB3, ErbB4, Erk, Fak, fes, FER, FGFR1, FGFR2, FGFR3, FGFR4, FGFR5, Fgr, flt-1, Fps, Frk, Fyn, GSG2, GSK, Hck, ILK, INSRR, IRAK4, ITK, IGF-1R, INS-R, Jak, KSR1, KDR, LMTK2, LMTK3, LTK, Lck, Lyn, MATK, MERTK, MLTK, MST1R, MUSK, NPR1, NTRK, MEK, MER, PLK4, PTK, p38, PDGFR, PIK, PKC, PYK2, RET, ROR1, ROR2, RYK, ros, Ron, SGK493, SRC, SRMS, STYK1, SYK, TEC, TEK, TEX14, TNK1, TNK2, TNNI3K, TXK, TYK2, Tyro-3, tie, tie2, TRK, Yes, and Zap70.
Protein tyrosine kinases are a subclass of protein kinase. They also may be classified as growth factor receptor (e.g. Axl, VEGFR, c-Met (HGFR), EGFR, PDGFR, and FGFR) or non-receptor (e.g. c-src and bcr-abl) kinases. Receptor tyrosine kinases are transmembrane proteins that possess an extracellular binding domain for growth factors, a transmembrane domain, and an intracellular portion that functions as a kinase to phosphorylate a specific tyrosine residue in proteins. Abnormal expression or activity of protein kinases has been directly implicated in the pathogenesis of myriad human cancers.
Angiogenesis, the formation of new capillaries from preexisting blood vessels, is a necessary process for organ development during embryogenesis and is critical for the female reproductive cycle, inflammation, and wound healing in the adult. Certain diseases are known to be associated with deregulated angiogenesis, for example ocular neovascularization, such as retinopathies (including diabetic retinopathy), age-related macular degeneration, psoriasis, hemangioblastoma, hemangioma, arteriosclerosis, inflammatory disease, such as a rheumatoid or rheumatic inflammatory disease, especially arthritis (including rheumatoid arthritis), or other chronic inflammatory disorders, such as chronic asthma, arterial or post-transplantational atherosclerosis, endometriosis, and neoplastic diseases, for example so-called solid tumors and liquid tumors (such as leukemias). Solid tumors, in particular, are dependent on angiogenesis to grow beyond a certain critical size by inducing new capillaries sprouting from existing blood vessels to secure their nutrition, oxygen supply, and waste removal. In addition, angiogenesis also promotes metastasis of tumor cells to other sites.
The new vessel growth and maturation are highly complex and coordinated processes, requiring the stimulation by a number of growth factors, but vascular endothelial growth factor (VEGF) signaling often represents a critical rate-limiting step in physiological angiogenesis and pathological angiogenesis. VEGF binds to and activates the receptor tyrosine kinase, VEGFR. Three VEGFR isoforms have been identified in humans: VEGFR-1 (Flt-1), VEGFR-2 (KDR/Flk-1) and VEGFR-3 (Flt-4). VEGFR-2 mediates the majority of cellular responses to VEGF, in particular its mitogenic and angiogenic effects. VEGFR-1 is thought to modulate VEGFR-2 signaling or to act as a dummy/decoy receptor to sequester VEGF away from VEGFR-2. The expression of VEGFR-1 is also up-regulated by hypoxia, in a similar mechanism to VEGF, via HIF-1; its functions may vary depending on cell type and developmental stage. (Stuttfeld E, Ballmer-Hofer K (September 2009). “Structure and function of VEGF receptors”. IUBMB Life 61 (9): 915-22.)
Since VEGFR-2 is the major mediator of vascular endothelial cell (EC) mitogenesis and survival, as well as angiogenesis and microvascular permeability, it is expected that direct inhibition of the kinase activity of VEGFR-2 will result in the reduction of angiogenesis and the suppression of tumor growth. Furthermore, inhibition of VEGFR-2 targeting the genetically more stable host endothelial cells, instead of labile tumor tissues, may decrease the chance of resistance development. Several agents targeting VEGFR signaling, administered either as single agents or in combination with chemotherapy, have been shown to benefit patients with advanced-stage malignancies. (“VEGF-targeted therapy: mechanisms of anti-tumor activity.” Nature Reviews Cancer, 2008, 8, 579; “Molecular basis for sunitinib efficacy and future clinical development.” Nature Reviews Drug Discovery, 2007, 6, 734; “Angiogenesis: an organizing principle for drug discovery?” Nature Reviews Drug Discovery, 2007, 6, 273).
c-Met, also referred to as hepatocyte growth factor receptor (HGFR), is expressed predominantly in epithelial cells but has also been identified in endothelial cells, myoblasts, hematopoietic cells and motor neurons. The natural ligand for c-Met is hepatocyte growth factor (HGF), also known as scatter factor (SF). In both embryos and adults, activated c-Met promotes a morphogenetic program, known as invasive growth, which induces cell spreading, the disruption of intercellular contacts, and the migration of cells towards their surroundings. (“From Tpr-Met to Met, tumorigenesis and tubes.” Oncogene 2007, 26, 1276; “Met Receptor Tyrosine Kinase as a Therapeutic Anticancer Target.” Cancer Letter, 2009, 280, 1-14).
A wide variety of human malignancies exhibit sustained c-Met stimulation, overexpression, or mutation, including carcinomas of the breast, liver, lung, ovary, kidney, thyroid, colon, renal, glioblastomas, and prostate, etc. c-Met is also implicated in atherosclerosis and lung fibrosis. Invasive growth of certain cancer cells is drastically enhanced by tumor-stromal interactions involving the HGF/c-Met pathway. Thus, extensive evidence that c-Met signaling is involved in the progression and spread of several cancers and an enhanced understanding of its role in disease have generated considerable interest in c-Met as major targets in cancer drug development. (“Molecular cancer therapy: can our expectation be MET.” Euro. J. Cancer, 2008, 44, 641-651; “Targeting the c-Met Signaling Pathway in Cancer.” Clin. Cancer Res. 2006, 12, 3657). Agents targeting c-Met signaling pathway are now under clinical investigation. (“Novel Therapeutic Inhibitors of the c-Met Signaling Pathway in Cancer.” Clinical Cancer Research, 2009, 15, 2207). “Drug development of MET inhibitors: targeting oncogene addiction and expedience.” Nature Review Drug Discovery, 2008, 7, 504).
Axl belongs to the subfamily of receptor tyrosine kinases (RTKs) that also includes Tyro3 and Mer (TAM). The TAM receptors are characterized by a combination of two immunoglobin-like domains and dual fibronectin type III repeats in the extracellular region and a cytoplasmic kinase domain. The ligands for TAM receptors are Gas6 (growth arrest-specific 6) and protein S, two vitamin K-dependent proteins that exhibit 43% amino-acid sequence identity and share similar domain structures (“The anticoagulation factor protein S and its relative, Gas6, are ligands for the Tyro 3/Axl family of receptor tyrosine kinases.” Cell, 1995, 80, 661-670; “Axl receptor tyrosine kinase stimulated by the vitamin K-dependent protein encoded by growth-arrest-specific gene 6.” Nature, 1995, 373, 623-626).
Adequate evidence supports the role of the Gas6/Axl system in driving cell growth and survival in normal and cancer cells (TAM receptor tyrosine kinases: biologic functions, signaling, and potential therapeutic targeting in human cancer. Adv Cancer Res 2008, 100, 35-83). Axl overexpression and signaling has been implicated in several human malignancies, such as colon, breast, glioma, thyroid, gastric, melanoma, lung cancer, and in renal cell carcinoma (RCC). A more detailed role of Axl biology has been proven in glioma, where loss of Axl signaling diminished glioma tumor growth, and in breast cancer, where Axl drive cell migration, tube formation, neovascularization, and tumor growth. Axl has been shown to play multiple roles in tumorigenesis and that therapeutic antibodies against Axl may block Axl functions not only in malignant tumor cells but also in the tumor stroma. The additive effect of Axl inhibition with anti-VEGF suggests that blocking Axl function could be an effective approach for enhancing antiangiogenic therapy. (“Axl as a potential therapeutic target in cancer: role of Axl in tumor growth, metastasis and angiogenesis.” Oncogene, 2009, 28, 3442-3455; “TAM Receptor Tyrosine Kinases: Biologic Functions, Signaling, and Potential Therapeutic Targeting in Human Cancer.” Adv Cancer Res. 2008, 100, 35-83).
It is widely known that cancer cells employ multiple mechanisms to evade tightly regulated cellular processes such as proliferation, apoptosis, and senescence. Thus, most tumors can escape from the inhibition of any single kinase. System-wide analyses of tumors identified receptor tyrosine kinase (RTK) coactivation as an important mechanism by which cancer cells achieve chemoresistance. One of the strategies to overcome RTK coactivation may involve therapeutically targeting multiple RTKs simultaneously in order to shut down oncogenic RTK signaling and overcome compensatory mechanisms. (“Receptor Tyrosine Kinas Coactivation Networks in Cancer.” Cancer Research, 2010, 70, 3857). Anti-tumor approaches in targeting VEGFR, c-Met and Axl signaling may circumvent the ability of tumor cells to overcome VEGFR, c-Met (HGFR) and/or Axl inhibition alone and thus may represent improved cancer therapeutics.